Justin Bogner: This makes the headers available in the build directory, making it
easier to use freshly-built clang with freshly-built libc++.

Basically, this makes it possible to run clang with libc++ without
having to install it, even if you don't have any version of libc++
installed in /usr/

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@194825 91177308-0d34-0410-b5e6-96231b3b80d8